The use of gas discharge lamps, such as high intensity discharge (HID) lamps, is common in a wide variety of applications. In a gas discharge lamp, light is emitted from an arc discharge established between lamp electrodes. Typically the lamp includes a lamp bulb including a sealed arc tube therein. The arc tube may enclose a fill material including, for example, one or more metal halides that are vaporized in the arc between the electrodes to establish the arc discharge. The fill may also include a buffer for limiting energy transfer from the arc discharge to the tube walls and, in some circumstances, a starting gas. The electrodes may be positioned at the top and bottom of the arc tube, and may be coupled to a ballast for generating the arc discharge in the fill material between the electrodes. The arc discharge between the electrodes emits light that passes through the light-transmissive materials of the tube and bulb for providing illumination.
It is known that under certain operating conditions the arc discharge may take an undesirable shape and/or become unstable. For example, operating a lamp with its axis in a horizontal position can result in an arc discharge which is bowed, or curved upwardly. In a non-horizontal position, the arc discharge may take a contorted shape, which may be unstable, and vapor phase segregation (incomplete mixing of the metal additives in the vapor phase of the discharge) may occur. These undesirable conditions in the arc discharge can lead to color separation over the length of the tube, unintended extinguishing of the arc discharge, reduced light output, local overheating of the arc tube wall, and other problems that may cause premature lamp failure. This may be particularly true for lamps having relatively high aspect ratio arc tubes, e.g. tubes whose length-to-width ratio is greater than about 2.
To reduce or avoid these conditions, modulation of the input lamp power at acoustic frequencies has been proposed. Modulation of lamp power causes modulation of the arc temperature distribution and, as a result, modulation of the gas pressure distribution throughout the arc discharge tube of the lamp. Certain frequencies of lamp power modulation cause standing wave acoustic oscillation of the gas pressure in the tube compelling gas or vapor movement to counter segregation or gravity-induced convection in the arc discharge.
The acoustic standing wave modes of the gas pressure in discharge lamps are known to those of skill in the art. In general, because of the generally cylindrical shape of commercial arc discharge tubes, the acoustic modes can generally be described as modes of a cylinder of a size comparable to the cavity in which the arc is formed in the arc tube of the lamp. If the pressure has a spatial dependence along the axis of the tube (i.e., the cylinder of comparable size), then the mode is longitudinal with the number of half-wavelengths in the standing wave determining the order of the mode. For example, if there are two half-wavelengths, the mode is the second longitudinal (2L) mode. If the pressure has a spatial dependence along the radius of the tube, then the mode is radial, and if the pressure has a spatial dependence around the circumference of the tube, then the mode is azimuthal. Combination acoustic modes are also possible, such as radial-longitudinal modes and azimuthal-longitudinal modes, in which the pressure distribution varies along more than one coordinate. These combination modes can be further defined, depending on the periodicity of the standing wave, such as a combination acoustic mode of the third azimuthal and second longitudinal modes.
The lamp power modulation frequencies for exciting any acoustic mode may be estimated using cylindrical models, as described for example in U.S. Pat. No. 6,844,687, the teachings of which are hereby incorporated herein by reference. Excitation of certain modes in the arc discharge tubes have been found to be particularly effective in avoiding associated conditions in the arc discharge. For example, exciting the second azimuthal (2A) mode may be particularly effective for straightening an arc discharge. Also, exciting the second longitudinal (2L) mode has been found to be particularly effective in reducing or avoiding segregation.